PRL secretion was determined in 63 children undergoing evaluation of GH status. Children were assigned to 1 of 3 groups based on GH studies: group 1, those with abnormal GH responses to provocative testing (n = 23); group 2, children with normal GH responses to provocative testing and mean 24-h GH concentrations below 2.5 micrograms/L (n = 14); or group 3, those with normal stimulated GH secretion and mean 24-h GH concentrations of 2.5 micrograms/L or more (n = 26). Serum PRL concentrations were measured in daytime (0800-1600 h), nighttime (2200-0600 h), and 24-h pools of serum specimens obtained every 20 min over a 24-h period. Mean (+/- SD) daytime (17.5 +/- 14.3 micrograms/L) and 24-h (19.2 +/- 13.0 micrograms/L) pool PRL concentrations were significantly higher in group 1 than in group 3 (daytime, 6.7 +/- 2.3; 24 h, 10.2 +/- 2.5 micrograms/L; P less than 0.01). Mean nighttime pool PRL concentrations did not differ among groups. Mean nighttime pool PRL values were significantly higher (P less than 0.01) than daytime pool values in group 3 (nighttime pool, 13.6 +/- 3.6 micrograms/L; night to day ratio, 2.2 +/- 1.0) and group 2 (16.8 +/- 9.0 micrograms/L; night to day ratio, 2.5 +/- 1.5), but not within group 1 (21.4 +/- 13.5 micrograms/L; night to day ratio, 1.4 +/- 0.5). The mean peak and increment in PRL concentrations after an iv bolus of insulin-TSH-LHRH were not different among groups. The mean decrement in serum PRL level after L-dopa ingestion was greater in group 1 than in group 3 (P less than 0.05). Two children in group 2 and 10 in group 1 had significantly elevated daytime pool PRL concentrations (greater than 11.3 micrograms/L; 2 SD above the mean value for group 3). Two additional children in group 2 and 2 in group 1 had elevated 24-h (greater than 15.2 micrograms/L) pool PRL concentrations. One child in group 2 and 3 in group 1 had low 24-h PRL concentrations (less than 5.2 micrograms/L; less than 2 SD below the mean of group 3). Fourteen of 20 children with elevated daytime and/or 24-h pool PRL levels or low 24-h pool PRL values had structural or radiation-associated insults to the hypothalamic-pituitary axis evident in the history or with brain-imaging techniques; 1 had microphallus with panhypopituitarism and 5 children had no structural abnormalities.(ABSTRACT TRUNCATED AT 400 WORDS)